1. Field of the Invention
The present invention relates to an electronic device which has a structure in which at least one electronic element including an insulating film or covered with a protecting film is formed on a substrate. The invention also pertains to a method for forming a film which can be used as the insulating or protecting film of the abovesaid electronic device.
2. Description of the Prior Art
During operation an electronic device of the abovesaid structure is accompanied by heat generation from the electronic element. In order to efficiently let heat escape from the electronic element, it is desirable that the protecting film be high in thermal conductivity.
Furthermore, it is desirable that the protecting film be highly resistant to alkali ions since it is degraded by alkali ions such as sodium ions, potassium ions, etc.
Moreover, it is desirable that the protecting film have as high a degree of electrical insulation as possible when it is formed an electrode, lead or like conductive layer of the electronic element in contact therewith.
Besides, when the electronic element is a programmable memory element which stores binary information "1" (or "0") through charging and stores the binary information "0" (or "1") through discharging of the stored charge by irradiation with ultraviolet rays having a wavelength of, for instance, 184 or 254 nm, it is desirable that the protecting film be highly permeable to the ultraviolet rays.
It is also desirable that the insulating film of the electronic device have a high degree of electrical insulation. This is true especially when the electronic device is an MIS transistor, bipolar transistor or like semiconductor element in which case a semiconductor substrate is used and the insulating film is formed to extend between the substrate and a conductive layer such as an electrode or lead, and when the electronic device is an MIS transistor in which case a semiconductor substrate is used and the insulating film is formed to extend between the substrate and a gate electrode.
For the same reason as that given above with respect to the protecting film, it is also desirable that the insulating film be highly resistant to the alkali ions.
Also it is preferable that the insulating film be highly permeable to ultraviolet rays when the electronic element is the aforementioned programmable memory element.
Heretofore there has been proposed an electronic device of the abovesaid structure in which the insulating film or protecting film is made of silicon nitride. The silicon nitride film is usually formed through a plasma CVD process using, as a material gas, a gas mixture containing silane (SiH.sub.4) and ammonia (NH.sub.3).
Such an insulating or protecting film made of silicon nitride has a relatively high degree of electrical insulation and is relatively high in resistance to alkali ions and permeable to ultraviolet rays having a relatively long wavelength of 254 nm.
However, this insulating or protecting film is so low in thermal conductivity that substantially no heat can be released.
Furthermore, since the insulating or protecting film of silicon nitride has a relatively small optical energy of about 5 eV, it is permeable to the ultraviolet rays having a relatively long wavelength of 254 nm but is not satisfactorily high in permeability and only slightly permeable to ultraviolet rays of a 185 nm wavelength.
Moreover, the insulating or protecting film of silicon nitride contains relatively large numbers of dangling bonds and clusters of silicon, and hence is not sufficiently high in electrical insulation.
In addition, the insulating or protecting film of silicon nitride is not sufficiently high in the resistance to alkali ions.
As described above, the conventional electronic device employing the silicon nitride film as the insulating or protecting film is defective in that it does not satisfy the above-mentioned requirements.